An de mieux analyser la dynamique près du seuil
ritique, nous pouvons également
examiner lesfor
es de depinningindividuelles:
σck(t) = σY(t, k) − σel(t, k) ,
(6.7)
où
k = 1, 2, ..., L
2
est l'indi
e de
haque site d'un système de taille
L
. La gure 6.7
représente la distribution de toutes les for
es de depinning individuelles
P (σck)
ave
, en
lignepointillée,leniveauduseuil
ritique
σ
∗
≈ 0.276
.Nouspouvons
lairementdistinguer
deux diérentes populations de sites. Les sites ayant une for
e de depinning supérieure
au seuil
ritique (
σck
> σ
∗
) forment la population majoritaire. De plus, les distributions
de leurs for
es de depinningparaissent similairespour diérentes tailles de système
L =
8, 16, 32, 64, 128, 256
.Enrevan
he, lesdistributionsdes for
es dedepinningdes sitessous-
ritiques (
σck
< σ
∗
) semblent dépendre de
L
.
Sur la gure 6.8, nous avons tra
é la proportion des sites sous-
ritiques
P (σck
< σ
∗
)
enfon
tionde latailledu système
L
.Lepoids relatifdelapartiesous-
ritiqueenfon
tion
de la tailledu système est dé
ritpar une loide puissan
e telle que:
P (σck
< σ
∗
) ∝ L−s
,
(6.8)
ave
s ≈ 1.34
. Ce
i nous permet de dénir une dimension fra
tale
ds
de l'ensemble des
sites sous-
ritiques :
P (σck
< σ∗) ∝ Lds/L2
,
(6.9)
ainsi,
ds
= s + 2 ≈ 0.66
. Il est à noter quela dimensionfra
tale pour uneligne élastique
est approximativement de
0.35
[90℄.
−2
−1
0
1
2
3
10−15
10−10
10−5
100
σck
P(
σ
ck
)
8x8
16x16
32x32
64x64
128x128
256x256
Figure6.7:Distributiondesfor
esdedepinningindividuelles
P (σck)
pourdiérentes
taillesdesystème
L = 8, 16, 32, 64, 128, 256
;lalignepointilléeindiqueleseuil
ritique
σ∗
: lapartie sous-
ritique (
σck
< σ
∗
) dépend delataille du système
L
.
101
102
10−4
10−3
10−2
10−1
L
P(
σ
ck
<
σ
*
)
L−1.34
Figure 6.8: Le poids relatif de la partie sous-
ritique
P (σck
< σ
∗
)
en fon
tion de
la taille du système
L
: dé
rit par une loi de puissan
e
P (σck
< σ
∗
) ∝ L−s
ave
s ≈ 1.34
.
sans pour autantavoirété "dépinglé"mais simplementrelaxéen raisondes
ontributions
négativesde lafon
tion de Green. Nouspourrons ainsi étudier, par exemple, lelien qu'il
peut yavoirentre letemps deséjour d'unsite, lenombre depas de tempsdurantlesquels
il reste dans la zone sous-
ritique avant de "dépingler" et/ou quitter
ette zone, et son
âge, le nombre de pas de tempsdepuis sa dernière a
tivation.
6.5 Con
lusion
Cara
térisé par une dynamiqueextrémale ave
des
onditions quasi-statiques,
e modèle
de plasti
ité nous a permis d'analyser les distributions des for
es de depinning. En fai-
santtendrelatailledu sous-systèmeétudiévers l'inni,lesdistributionstendent vers une
distributionDira
entréeàun seuil
ritique
σ
∗
.Ce dernierpeutêtre déniparl'extrapo-
lation de l'é
art type des for
es de depinningen fon
tion de leur moyenne, de telle façon
quele
roisemententre
ette extrapolationetl'axede lamoyenne représente
σ
∗
. Ensuite,
nous avons tenté de superposer
es distributions sur une
ourbe maîtresse, révélant une
formeuniverselle,maislaremiseàl'é
helleutilisées'estavéréeinsusante. Parallèlement,
nous avons pu déterminer la proportion des sites sous-
ritiques par l'étude des for
es de
depinningindividuelleset ainsi dénirleur dimension fra
tale
ds≈ 0.66
.
Con
lusion et perspe
tives
Partantdel'hypothèsequeladéformationplastiquema
ros
opiquedesmatériauxamorphes
estunesu
essionderéorganisationsindividuellesetlo
alisées,nousavonsdéveloppépour
e travailde thèse un modèle mésos
opique,appartenant àlafamilledes modèles de pin-
ning, pour étudier l'eet des intera
tions élastiques induites par
es réarrangements sur
le
omportementplastique.Après avoirprésenté lemodèle etlasolutionanalytiquede la
réponse élastiqueà unedéformationplastique en
isaillementd'unein
lusionau
hapitre
1, nous nous sommes
on
entrés au
hapitre 2 sur le passage dis
ret/
ontinu de
ette
dernière qui demeure un point
lé ettrès sensible aux détailsnumériques.
Ce modèle nous a permis en premier lieu de retrouver un
omportement ma
ros
o-
pique
omparableà
eluiobtenulorsd'untest mé
aniqueetde fournirune interprétation
statistique pour le phénomène d'é
rouissage au
hapitre 3. Au
hapitre 4, nous avons
pu démontrer que les événements su
essifs sont fortement
orrélés et ainsi mettre en
éviden
ela lo
alisationde
es événements ave
une anisotropie des positionnementsrela-
tifs,
ara
tériséepar lamêmesymétriequadripolairequelafon
tionde redistributiondes
ontraintes élastiques.Au-delàdu
omportementdumodèleentantquesimplesu
ession
de réarrangements individuels et lo
alisés, nous avons étudié le phénomène d'avalan
he
d'événementsplastiquesau
hapitre5.Le
hapitre6,quantàlui,aété
onsa
réàl'analyse
des for
esdedepinningauvoisinagedeseuil
ritiqueetla
ara
térisation delaproportion
des sites sous-
ritiques.
Les résultatsde
e modèle s
alaire,basé sur la"
ompétition"entre désordre et inter-
a
tions élastiques,suggèrent plusieurs perspe
tives :
- Dans les simulations présentées, nous avons imposé aux déformations lo
ales la
mêmedire
tion que ladéformationma
ros
opiqueen
isaillementdu système. An
d'ajouter un degré de liberté et ainsi espérer mieux analyser les réarrangements
lo
aux, un désordre dire
tionnel peut être implémentédans le modèle.
- Pour une des
ription plus réaliste de la plasti
itédes amorphes,
e modèle, qui est
s
alaire, doit prendre en
ompte l'aspe
t tensoriel du problème. Il est essentiel de
permettre ausystèmedes modi
ationslo
ales tanten termede
isaillementquede
volume. Ce
i
onstitue sans doute un point très important pour la
ompréhension
des mé
anismes de déformation.
- Pour pouvoir atteindre des systèmes de grande taille et ainsi parfaire les analyses
d'un tel modèle mésos
opique, il est envisageable de développer une version ave
réorganisation,ledéveloppementd'un pro
édé permettant une
ara
térisation plus
nede
esréarrangementsave
dessimulationsdetypedynamiquemolé
ulairepeut
être suggéré.
Ce modèle statistique à l'é
helle mésos
opique, a priori basé sur des ingrédients très
simples, ouvre don
divers
hamps de travailintéressants.
Annexe A
Identi
ation d'événements plastiques
lo
alisés par intégrale de
ontour
Partantduprin
ipemé
aniqueélémentairedeszonesderéorganisationstru
turelle("shear
transformation zones",STZ)pourdé
rirelaplasti
itédesmatériauxamorphes,une ques-
tion
ru
ialeest :
ommentidentieretanalyser
es régions?Plusieursétudesnumériques
ontré
emmentétéréaliséespouridentier
esévénementsplastiquesélémentaireslo
alisés
par des simulationsen dynamiquemolé
ulaire de matériauxamorphes sous
isaillement:
ave
des verres Lennard-Jones[25, 24℄ etdes verres métalliques [54℄, par exemple.
De manière analogue à l'intégrale
J
de Ri
e [91℄, développée pour estimer l'intensité
d'une singularité d'un
hamp de
ontrainte autour d'un front de ssure, l'obje
tif i
i est
de
apter les singularités du
hamp de
ontrainte induit par les déformations plastiques
lo
ales.À grandedistan
e,
es réorganisationspeuvent êtretraitées
ommedes in
lusions
d'Eshelby [78℄. Nous développons en deux dimensions une appro
he simple basée sur le
formalismede Kolossov-Muskheli
hvili[79℄ du plan élastique.
Ce formalismenous permetde réé
rire, par une expansionen loide puissan
e des va-
riablesduplan
omplexe,les
hampsdedépla
ementetde
ontrainteélastiqueinduitspar
une in
lusionplastique. Loindu
entre de lazonedéformée, les
ontributions dominantes
sont asso
iées au premier ordre des singularités, de symétrie dipolaire ou quadripolaire,
orrespondant respe
tivement à une déformation plastique volumique pure ou à une dé-
formation déviatorique pure d'une in
lusion
ir
ulaire. Par la
onstru
tion de fon
tions
holomorphes à partir du
hamp de dépla
ement et de ses dérivées, il est possible de dé-
nir une intégrale de Cau
hy au
ontour indépendant an de
apter l'amplitude de
es
singularités.
Les expressions analytiques ainsi que des tests numériques, basés sur des simulations
par élémentsnis,sont présentés danslapubli
ationquenous avons miseen piè
e jointe.
Malgrélasatisfa
tiondespropriétésd'orthogonalitéetd'indépendan
evisàvisdu
ontour
hoisi, la robustesse numérique de
ette méthode s'avère faible en raison de la né
essité
d'évaluer lesdérivées du
hamp de dépla
ement.
Path-independent integrals to identify localized plastic events in two dimensions
Mehdi Talamali,1Viljo Petäjä,1Damien Vandembroucq,1,2and Stéphane Roux3
1Unité Mixte CNRS–Saint-Gobain “Surface du Verre et Interfaces”, 39 Quai Lucien Lefranc, 93303 Aubervilliers cedex, France
2
Laboratoire PMMH, ESPC, CNRS, Paris 6, Paris 7, 10 rue Vauquelin, 75231 Paris cedex 05, France
3
LMT-Cachan, ENS de Cachan, CNRS-UMR 8535, Université Paris 6, PRES UniverSud, 61 avenue du Président Wilson, F-94235
Cachan cedex, France
sReceived 28 January 2008; revised manuscript received 2 June 2008; published 22 July 2008d
We use a power expansion representation of plane-elasticity complex potentials due to Kolossov and
Muskhelishvili to compute the elastic fields induced by a localized plastic deformation event. Far from its
center, the dominant contributions correspond to first-order singularities of quadrupolar and dipolar symmetry
which can be associated, respectively, with pure deviatoric and pure volumetric plastic strain of an equivalent
circular inclusion. By construction of holomorphic functions from the displacement field and its derivatives, it
is possible to define path-independent Cauchy integrals which capture the amplitudes of these singularities.
Analytical expressions and numerical tests on simple finite-element data are presented. The development of
such numerical tools is of direct interest for the identification of local structural reorganizations, which are
believed to be the key mechanisms for plasticity of amorphous materials.
DOI:10.1103/PhysRevE.78.016109
PACS numberssd: 62.20.F2, 46.15.2x, 02.30.Fn, 81.05.Kf
I. INTRODUCTION
The plasticity of amorphous materials has motivated an
increasing amount of study in recent years. In the absence of
an underlying crystalline lattice in materials such as foams,
suspensions, or structural glasses, it is generally accepted
that plastic deformation results from a succession of local-
ized structural reorganizationsf1–4g. Such changes of local
structure release part of the elastic strain to reach a more
favorable conformation and induce long-range elastic fields.
The details of such local rearrangements and of the internal
stress they induce obviously depend on the precise structure
of the material under study, and its local configuration. How-
ever, the important observation is that outside the zone of
reorganization a linear elastic behavior prevails. Therefore,
elastic stresses can be decomposed onto a multipolar basis
and, independently of the material details, it is possible to
extract singular, scale-free, dominant terms which can be as-
sociated with a global pure deviatoric or pure volumetric
local transformation of an equivalent circular inclusion. In
particular, the elastic shear stress induced by a localized plas-
tic shear exhibits a quadrupolar symmetry. This observation
has motivated the development of statistical models of amor-
phous plasticity at the mesoscopic scale based upon the in-
teraction of disorder and long-range elastic interactions
f5–8g. In the same spirit, statistical models were also recently
developed to describe the plasticity of polycrystalline mate-
rialsf9g. Several numerical studies have been performed re-
cently to identify these elementary localized plastic events in
athermal or molecular dynamics simulations of model amor-
phous materials under shearf10,11g.
The question remains of how to identify and analyze these
transformation zones. In analogy with the path-independent
Rice J integralf12g developed to estimate the stress intensity
factor associated with a crack tip stress singularity, we aim
here at capturing the stress singularity induced by the local
plastic transformation which can be treated as an Eshelby
inclusionf13g. In two dimensions, we develop a simple ap-
proach based upon the Kolossov-Muskhelishvili
sKMd for-
malism of plane elasticityf14g. This is an appealing pathway
to the solution since these zones will appear as poles for the
potentials, and hence Cauchy integrals may easily lead to
contour integral formulations which are independent of the
precise contour geometry, but rather rely on its topology with
respect to the different poles that are present.
Although these techniques have been mostly used in the
context of numerical simulations in order to estimate stress
intensity factors from finite-element simulations, they are
now called for to estimate stress intensity factors from ex-
perimentally measured displacement fields from, e.g., digital
image correlation techniques. In this case, interaction inte-
gral techniques
f15g or least squares regression f16g tech-
niques have been applied. Noise-robust variants have also
been proposedf17g. These routes could also be followed in
the present case.
Though the present work is restricted to two dimensions
due to the complex potential formulation, similar questions
can be addressed for the three-dimensional version of this
problem using the same strategy but a different methodology.
In the following, we briefly recall the KM formalism, we
give analytic expressions for the contour integrals allowing
us to capture the singular elastic fields, and we present a few
numerical results based on a finite-element simulation sup-
porting our analytical developments.
In Sec. II, we present the theoretical basis of our approach
in terms of singular elastic fields, while in Sec. III we intro-
duce the contour integral formulation. In Sec. IV, a numerical
implementation based on finite-element simulations is pre-
sented, together with the results of the present approach. This
application allows us to evaluate the performance and limi-
tations of the contour integral procedure and check the det-
rimental effect of discreteness. Section V presents the main
conclusions of our study.
II. POTENTIAL FORMULATION
In two dimensions, the Kolossov-Muskhelishvili poten-
tials can be used to write the elastic stress and displacement
PHYSICAL REVIEW E 78, 016109s2008d
duce the elastic displacement U = Ux+ iUyand the stress ten-
sor field through two functions, the real trace S0= sxx+ syy
and the complex function S = syy− sxx+ 2isxy. In the frame-
work of linear elasticity, the balance and compatibility equa-
tions can be rewritten as
S0,z− S,z¯= 0,
s1d
S0,zz¯= 0,
s2d
where z = x + iy is the complex coordinate and the notation A,x
is used to represent the partial derivative of field A with
respect to coordinate x. Note that we assumed zero surface
density force and that Eq.s2d is here the classical Beltrami
equation which expresses the kinematic compatibility condi-
tion in terms of stress. The general solution to these equa-
tions can be obtained through the introduction of two holo-
morphic functions w and
c, called the KM potentials. The
displacement and the stress field can be writtenf14g
2mU =kwszd − zw8szd −cszd,
s3d
S0= 2fw8szd + w8szdg,
s4d
S = 2fz¯w9szd +c8szdg,
s5d
where
m
is the elastic shear modulus and
k=s3 − 4nd for
plane strain ands3 − nd / s1 + nd for plane stress, n being the
Poisson ratio.
III. PLASTIC INCLUSION AND SINGULARITY
APPROACH IN TWO DIMENSIONS
A. Singular terms associated with plastic inclusion
This KM formalism can be applied to two-dimensional
inclusion problems
f18,19g. Let us consider the case of a
small inclusion of area A experiencing plastic deformation
and located at the origin of the coordinate system z = 0. It is
assumed that the stress is a constant at infinity. Outside the
inclusion, the KM potentials can be expanded as a Laurent
series as
wszd =aoutz+o
n=1
`
wn
zn,
cszd =b
outz+o
n=1
`
cn
zn.
s6d
The linear terms can be easily identified as corresponding to
uniform stresses while constant termssomitted hered would
lead to a rigid translation. It can be shown in addition that the
dominant singular terms w1/z
and
c1/z
can be associated
with the elastic stress induced by the plastic deviatoric and
volumetric strain of an equivalent circular inclusion of area
A. That is, considering a circular inclusion experiencing a
plastic shear straingpand a plastic volumetric strain
dpwe
havef18g
w1=
2imAgp
psk+ 1d,
c1= −
2mAdp
psk+ 1d.
s7d
In particular, for a pure shear plastic event we obtain a qua-
drupolar symmetry:
sxy= −
k+ 1pr2coss4ud.
s8d
Note that we have in general to consider a complex value of
gpto include the angular dependence of the principal axis. In
contrast, the amplitude
c1
is a real number
snote that the
imaginary part would correspond to a pointlike torque ap-
plied at the origind.
B. Generic character of the expansion
Because of the well-known property of Eshelby circular
inclusion, the above expansion limited to the
c1
and
f1
terms only is the exactsouterd solution of a uniform plastic
strain distributed in the inclusion, and vanishing stress at
infinity. However, one should note that this result is much
more general. Indeed, it is seen that the physical size of the
inclusion does not enter into the solution except through the
products Agpand Adp. Therefore, a smaller inclusion having
a larger plastic strain may give rise to the very same field,
provided the products remain constant. Thus one can con-
sider the prolongation of the solution to a pointlike inclusion
swith a diverging plastic straind as being equivalent to the
inclusion.
Then from the superposition property of linear elasticity, a
heterogeneous distribution of plastic strain
gpsxd in a com-
pact domain D will give rise to such a singularity with an
amplitude equal to
fAgpgeq=EE
D
gpsxddx
s9d
and the same property would hold separately for the volu-
metric part. As a particular case, one finds a uniform plastic
strain for an inclusion of arbitrary shape.
This is the key property that allows us to capture the
equivalent plastic strain of an arbitrary complex configura-
tion, for the above mentioned application to amorphous me-
dia. In fact, this is even the only proper way of defining the
plastic strain for a discrete medium as encountered in mo-
lecular dynamics simulations. The far-field behavior of the
displacement and stress field can be accurately modeled, and
without ambiguity, by a continuum approach, and thus the
above result will hold. In contrast, locally, the large-scale
displacement of several atoms may render difficult the direct
computation of the equivalent plastic strain experienced in
such an elementary plastic event.
Let us, however, stress one difficulty: As the above argu-
ment ignores the details of the action taking place within the
“inclusion,” plasticity has to be postulated. However, a dam-
aged inclusion, where the elastic moduli have been softened
by some mechanism, or even a nonlinear elastic inclusion at
one level of loading, would behave in a similar way to the
above plastic inclusion. Obviously, to detect the most rel-
evant physical description, one should have additional infor-
mation, say about unloading. If the above amplitudes remain
constant during unloading, plasticity would appear appropri-
ate. If the amplitude decreases linearly with the loading, then
damage is more suited. Finally, if the amplitudes varies re-
versibly with the loading, nonlinear elasticity might be the
best description. Thus, although one should be cautious in
the interpretation, local damage detection from the far field
may also be tackled with the same tools.
C. Path-independent contour integrals
In two dimensions, this multipole expansion formalism in
the complex plane suggests resorting to contour integrals to
extract the singularities. However, the displacement field is
not a holomorphic function and cannot be used directly for
that purpose. The strategy of identification of the singulari-
ties wnandcnthus consists of expressing the potentials from
the displacement field and its derivatives in order to extract
the singularities via Cauchy integrals. We now simply ex-